The invention relates to a method of recording image data on a record medium.
Imagesetters are well known for recording image data on photographic film or plates, each pixel on the plate being exposed to a radiation beam such as a laser beam having an intensity modulated in accordance with the colour component content of the pixel.
A variety of devices are known including flat bed and cylinder based apparatus.
In these apparatus, a radiation beam which has been modulated in accordance with the image data, is transmitted towards a rotating reflector which reflects the modulated beam onto the record medium. In order to increase the speed of imaging, it is well known to utilize more than one radiation beam so that two or more scan lines can be exposed simultaneously. See for example U.S. Pat. No. 5,502,709.
There is a need to be able to increase the resolution i.e. number of pixels per unit dimension, which can be exposed on the record medium. An obvious solution to this is to slow down the rotation speed of the reflector which will allow increased spatial resolution to be achieved on the record medium. However, conventional reflectors or spinners rotate at very high speed, for example of the order of 40,000 rpm and are mounted on air bearings which are designed to handle this speed. Consequently, it is difficult to adjust the rotational speed of the spinner since this would also require adjustment of the bearing structure.
In accordance with the present invention, a method of recording image data on a record medium comprises causing at least two radiation beams to impinge on a rotating reflector such that the beams are reflected onto a record medium and scanned across the record medium as the reflector rotates; modulating the radiation beams with image information; and causing relative movement between the reflector and the record medium in a direction parallel with the rotational axis of the reflector so that the radiation beams expose pixels on the record medium spaced apart along respective scan lines, the method further comprising adjusting the angle at which at least one of the beams is incident on the reflector as the reflector rotates such that data defining the image content of successive pixels to be exposed is used to modulate each beam in turn.
We have realised that it is possible to utilize a multi-beam scanner such as a multi-beam imagesetter and instead of arranging the beams simultaneously to expose lines of parallel pixels, we use two (or more) beams to expose successive pixels in turn and thus achieve much higher resolutions without having to vary rotation speeds. By xe2x80x9csuccessive pixelsxe2x80x9d we mean successive in the fast scan or rotational direction. Indeed, one of the significant advantages of this invention is that conventional apparatus used for multi-beam purposes can be very simply adjusted to effect the invention.
In some cases the scan lines could be partly or completely offset laterally i.e. along the axis of rotation. Preferably, however the angle at which at least one of the beams is incident on the reflector as the reflector rotates is chosen so that the scan lines coincide on the record medium to define a common scan line and the pixels exposed by each beam are interleaved along the common scan line.
Although the invention can be implemented with just two radiation beams, in the preferred example three radiation beams are used. This enables the resolution to be tripled without changing the rotational speed of the reflector.
Typically, the rate of relative movement between the reflector and the record medium in a direction parallel with the rotational axis will be slowed down from the conventional rate in order to equalise the pixel spacing in both directions.